The present invention relates to a metal printed circuit board used for a power converting device, such as an inverter.
The metal printed circuit board is superior to an ordinary glass epoxy resin printed circuit board in that heat generated from circuit parts mounted on the board can readily be radiated. However, the former is inferior to the latter in that signal and noise leakage are apt to occur because of a relatively large stray capacitance between the metal sheet and the conductor circuit.
FIG. 3 is a sectional view showing a general structure of a conventional metal printed circuit board. In the FIGURE, reference numeral 1 designates a metal plate and numeral 2 an insulating plate. Usually, the insulating plate 2 is attached to the metal plate 1. A circuit conductor 3 is formed on the insulating plate 2. Electronic parts 4 are mounted on the circuit conductor by means of solder 5, for example. A stray capacitance 6 formed between the insulating plate 2 and the metal plate 1 is symbolically illustrated.
Most of heat generated from the electronic parts 4 is radiated in the direction of arrows 8. The thermal conduction varies inversely with the thickness t1 of the insulating plate 2. Generally, the thickness t1 of the insulating sheet 2 is chosen to be as thin as possible, without sacrificing the ability of the sheet to withstand the voltage applied between the conductor circuit 3 and the metal plate 1.
Signal leakage possible occurs between the conductor circuits, through stray capacitance. External noise possible leaks into the circuits through the metal plate 1. One of the measures taken to solve the problems of the signal and noise leakage has been to increase the thickness t1 of the insulating plate 2. This solution however, results in loss of thermal conduction and in satisfactory heat radiation.
The problem to be solved by the invention is reduction in stray capacitance between the circuit conductor and the metal sheet on the metal printed circuit board while maintaining good heat radiation.